Generally, broadcasting systems such as NTSC (National Television System Committee), PAL (Phase Alternation by Line), and SECAM (Sequence De Couleurs Avec Memoirs) are used.
Japanese Patent Laid-Open No. Hei-4-103288 (entitled "NTSC/PAL Signals Discriminating Circuit, And Active Filter Using This Discriminating Circuit") discloses a technique as described below. Large signals are generated in correspondence with an input signal, and then, based on the magnitude of the generated large signals, a discrimination is made as to whether the input signal is an NTSC sub-carrier signal or a PAL carrier signal. Then the reference discloses the technique of carrying out adjustments for the required processing under the respective NTSC and PAL modes.
However, the PAL system includes the PAL-M system, PAL-N system, PAL-B system, and PAL-I system which have different functions.
Therefore, in a number of countries in which the television broadcasting is made based on the PAL-M system and the PAL-N system, a receiving system capable of receiving television signals based on the NTSC system, PAL-M system and PAL-N system was developed in order to receive the NTSC signals transmitted from the nearby countries.
However, in such a receiving system, a mechanical mode selecting switch such as a slide switch is used in selecting the modes of the NTSC system, PAL-M system and PAL-N system, since a circuit for automatically discriminating from among the three types of broadcasting systems is not known. Therefore there is the problem that users have to switch manually from among the modes by manipulating the mode selecting switch.